The present application claims priority to Japanese Application No. P10-249880 filed Sep. 3, 1998 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a beam irradiation apparatus, an optical apparatus having a beam irradiation apparatus for an information recording medium, a method for manufacturing an original disk for an information recording medium, and a method for manufacturing an information recording medium.
2. Description of the Related Art
As information recording media, there are CD-ROMs (Compact Diskxe2x80x94Read Only Memories), so-called CD-Rs, and recordable disks such as magneto optical disks. FIG. 8 shows a perspective view of such a recording medium. Each of FIGS. 9A and 9B is a schematic oblique view of a principal part showing sections of a part of an optical disk. On a signal recording region 2 formed on a disk substrate 1, minute concaves and convexes 18 are formed as shown in FIGS. 9A and B. The minute concaves and convexes 18 are formed by, for example, continuous grooves 3 as shown in FIGS. 9A. Alternatively, a sequence of pits 4 are spirally formed on each of tracks having a predetermined track pitch, for example, in the range of 1 xcexcm to 2 xcexcm.
On a signal recording surface having the minute concaves and convexes formed thereon, a light reflection film or a protection film is formed, in the case of, for example, CD-ROMS. In the case of recordable optical disks of phase change type, magneto optical recording type or the like, a recording layer using a phase change film or a magnetic film is formed on the signal recording surface having the minute concaves and convexes formed thereon, and a light reflection film and a protection film are formed thereon.
For example, in the above described recordable optical disks, the so-called lands interposed between the grooves 3 are typically used as a recording section, and the grooves 3 are used as a light reflection section for tracking.
Reproduction from such an information recording medium such as an optical disk and optical recording onto a recordable optical disk are conducted by irradiating a laser light typically from a rear surface of the substrate 1 opposite to the recording surface while rotating the disk.
For example, in the CD-ROM used exclusively for reproduction, information readout and tracking are conducted by detecting a reflected light or diffracted light coming from the pit 4.
In the recordable optical disk, information is optically written (recorded) on the recording layer located, for example, on the land portion by irradiation a laser light. By using a reflected light, readout (reproduction) of the recorded information is conducted. In order to irradiate a laser light for recording or reproduction always onto a predetermined track, a reflected light from, for example, the groove 3 is detected and tracking is effected.
As for the formation of the above described minute concaves and convexes, they are molded simultaneously with the molding of the disk substrate 1 by, for example, the injection molding. Alternatively, for example, an ultraviolet ray setting resin layer is formed on the disk substrate 1 by coating, and minute concaves and convexes are formed thereon. The latter method is the so-called 2P method (Photo Polymerization method) or the like.
In either of the injection molding method and the 2P method, a stamper having minute concaves and convexes inverted in pattern as compared with the minute concaves and convexes formed on the disk substrate 1 is used. For example, in the case of the injection molding, the stamper having the above described inverted pattern is disposed in a cavity of a molding die, and a disk substrate having concaves and convexes transferred from the stamper is molded by the injection molding of resin. In the case of the 2P method, there are included the steps of pressing the stamper against, for example, an ultraviolet ray setting resin layer coated onto the disk substrate, forming thereby minute concaves and convexes, then conducting cure using irradiation of ultraviolet rays, and thus forming a disk having minute concaves and convexes transferred from the stamper.
A method for fabricating such a stamper having minute concaves and convexes formed thereon will now be described by referring to perspective views in respective steps shown in FIG. 10A to FIG. 10C. In this case, an original disk is first fabricated. As for the fabrication of this original disk, a disklike substrate 11, such as a glass substrate, serving as a substrate of the original disk and having a smoothed mirror surface is prepared as shown in FIG. 10A.
Onto the smooth mirror surface of this substrate 11, a photoresist layer 12 containing, for example, positive photoresist is coated by using the rotation coating method or the like so as to have a required thickness, such as a thickness of 0.1 xcexcm as shown in FIG. 10B.
Then, this photoresist layer 12 is objected to a required pattern exposure. In other words, while the substrate 11 is being rotated around its center axis as shown in FIG. 10C, a laser beam 20 is focused onto the photoresist layer 12 by a focusing lens 21, and irradiated while it is relatively moved in the radial direction of the substrate 11. A latent image of minute concaves and convexes, i.e., a latent image 22 of a groove or pit is formed along a spiral line.
The photoresist layer 12 thus objected to the pattern exposure is developed. By doing so, there is obtained an information recording medium having minute concaves and convexes 13 each comprised of a groove or a pit formed by removing the photoresist layer 12 according to a predetermined exposure pattern. In this example, an original disk 14 for manufacturing optical disks is obtained. FIG. 11A or B shows a partial perspective view thereof.
By using the original disk 14 thus formed, a stamper is fabricated. As for the fabrication of the stamper, a metal layer 15 is formed on the surface of the original disk 14 having minute concaves and convexes 13 formed thereon by using a Ni plating as shown in FIG. 12A. The metal layer 15 is exfoliated from the original disk 14. By doing so, a stamper 17 comprised of the metal layer 15 having minute concaves and convexes 16 formed by inverting in pattern the minute concaves and convexes 13 of the original disk 14 is formed as shown in FIG. 12B.
By using the stamper 17 thus formed, it is possible as shown in FIGS. 13A and 13B to obtain an optical disk, i.e., an information recording medium 19, having minute concaves and convexes 18 inverted in pattern as compared with the minute concaves and convexes 16 of the stamper 17, i.e., having grooves 3 and pits 4 shown in FIG. 9A and FIG. 9B, by using the above described injection molding or 2P method.
In this case, the stamper 17 has been formed from the original disk 14 by the Ni plating. An alternative method includes the steps of fabricating a so-called master from the original disk 14 by the Ni plating, fabricating a so-called mother by transferring the master, and fabricating a stamper by transfer from the mother.
An exposure apparatus, i.e., a beam irradiation apparatus, used in the exposure process for the photoresist layer 12 has a schematic configuration as shown in FIG. 14. There is provided a beam generation source 31 for generating a laser light, to which the photoresist layer 12 is exposed. The laser light emitted from the beam generation source 31 is focused and irradiated on the photoresist layer 12 of the substrate 11 to be exposed, via a mirror 32, a condenser lens 33, a modulator 34, a collimator lens 35, mirrors 36 and 37, and an objective lens, i.e., a focusing lens 21.
With respect to the irradiation object, the beam irradiation apparatus conducts a light intensity modulation of the beam, or concretely turns on and off the beam, by using the modulator 34 according to the latent image of the minute concaves and convexes to be finally obtained, i.e., corresponding to the exposure pattern. As this modulator 34, an acousto-optic modulator (AOM) using, for example, an optical crystal can be employed.
In the modulator 34 using the AOM, a compressional wave of a refractive index is formed in the optical crystal by making an ultrasonic wave incident on the optical crystal. With respect to a diffraction grating formed by the compressional wave, an incident laser beam which has been incident at an angle satisfying the Bragg condition is objected to Bragg diffraction. There is thus obtained, for example, a primary diffracted light diffracted with a predetermined diffraction angle with respect to 0th light transmitted as a transmitted light as it is. The light intensity of this diffracted light depends upon the intensity of the ultrasonic wave which is incident on the crystal. In other words, selection as to whether or not the diffracted light is present can be conducted by turning on or off the ultrasonic wave. By using this primary diffracted light as the exposure laser beam, therefore, it is possible to conduct intensity modulation of the exposure laser beam, i.e., turn on or off the exposure laser beam according to control of the ultrasonic wave, i.e., according to turning on or off the ultrasonic wave.
As the recording density of information recording media such as optical disks is increased in recent years, how small the above described minute concaves and convexes, i.e., pits and grooves are formed has become a great problem.
In one method of solving this problem, the exposure spot diameter, i.e., the condensed spot diameter is made small. This spot diameter xcfx86 is given by
xe2x80x83xcfx86=1.22xcex/N.A
where N.A. is the numerical aperture of the focusing lens (objective lens), and xcex is the wavelength of light. For making the spot diameter xcfx86 small, therefore, a light having a short wavelength is used, or a focusing lens having a large N.A. is used.
As for the N.A. among them, the refractive index around the condensed point can be made large by employing a focusing lens using a SIL (Solid Immersion Lens). Therefore, its N.A. can be made equal to or more than 1.0.
In this SIL, one of its principal faces is, for example, a hemispherical face, and the other of its principal faces is a plane. By making a laser beam incident on the hemispherical face, a condensed spot can be formed on the plane.
Since the refractive index at the condensed point becomes the refractive index of the SIL, the diameter of the condensed spot can be made sufficiently small. If, for example, a laser beam focused by a lens having an N.A. of 0.9 is incident on the hemispherical face of the SIL having the reflective index of 2.0 while maintaining the focusing angle, then a spot formed on its plane side becomes equivalent to that focused by a lens having a N.A. of 1.8. In other words, the diameter of the condensed spot can be made small by using the SIL.
When a condensed spot is formed outside the plane side of the SIL in the case where a laser beam is condensed by using this SIL, however, a component having a large focusing angle included in the focused beam is totally reflected on this plane and is not emitted to the outside. In this case, therefore, the condensed spot formed outside the SIL becomes the same as that of a lens having a N.A. equal to or less than 1.0. In the case where the SIL is used, a light irradiation object such as the exposed surface must be kept in such a position that physically it is not in contact with the plane face of the SIL, but optically it is in contact therewith. In other words, the exposed surface must be kept in such a position that the amplitude of the optical wave does not become {fraction (1/10)} in the so-called near-by field region, i.e., in the light oozing region for the plane. In other words, the exposed surface must be kept in such a position that the distance between the exposed surface and the plane face of the SIL is equal to or less than                     λ        ·        log            ⁢              xe2x80x83            ⁢      e10              2      ⁢      π      ⁢                                                  (                              N                .                                  xe2x80x83                                ⁢                A                .                            )                        2                    -          1                      ⁢      xe2x80x83  
For example, when the N.A. is 1.8, the exposed surface must be disposed in such a position that an extremely minute distance equal to approximately one fourth of the wavelength xcex is kept.
When a beam having xcex of approximately 400 nm is used, the distance equal to one fourth of the light wavelength is approximately 100 nm. In the configuration of the conventional beam irradiation apparatus such as the exposure apparatus, it is extremely difficult to stably keep the interval between the focusing lens and the irradiation object at such a distance.
For example, in such a configuration that the above described SIL is attached to a servo actuator using an electromagnetic coil and focus servo is applied, it is now assumed that the servo gain thereof is approximately 40 dB and the substrate 11 having the irradiation object such as the photoresist layer 12 fluctuates by several xcexcm. In this case, focusing remainder amounts to several tens nm. It is thus impossible to effect focus control stably with the distance between the exposed face and the plane face of the SIL being equal to approximately 100 nm as described above.
As a configuration for keeping the distance between the SIL and the irradiation object at a small value, there can be adopted an air slider configuration of dynamic pressure type in which the SIL is floated from the irradiation object by wind pressure caused by rotation of the irradiation object. In this case, a change of the rotation speed of the irradiation object causes a variation of the distance between the SIL and the irradiation object and a risk of collision between them.
As the beam irradiation apparatus, the exposure apparatus for fabricating an original disk which is used to obtain an information recording medium has been mainly described. However, there is the same problem also in a beam irradiation apparatus included in an optical apparatus for recording and/or reproducing information onto and/or from a high density information recording medium such as the above described optical disk, i.e., included in the so-called pickup apparatus.
The present invention enables a reliable and stable focus control even in the case where the irradiation object is disposed near the above described focusing lens using, for example, the SIL to such an extent that the irradiation object is disposed in the near-by field region.
In other words, an object of the present invention is to provide a beam irradiation apparatus capable of providing a minute beam spot, an optical apparatus having such a beam irradiation apparatus for an information recording medium, a method for fabricating an original disk for an information recording medium capable of thereby effecting high density recording, and a method for manufacturing an information recording medium.
According to an aspect of the present invention, there is provided a beam irradiation apparatus which includes a beam generation source for generating at least either of an optical beam, an electron beam and an ion beam, and a focusing lens for focusing a beam emitted from the beam generation source, in which the beam irradiation apparatus includes at least first and second focus control mechanisms each controlling a focus position of the beam with respect to the focusing lens or the same lens system forming the focusing lens in an overlapping fashion, so as to focus the beam onto an irradiation object by using the focusing lens.
Furthermore, according to another aspect of the present invention, there is provided an optical apparatus having a beam irradiation apparatus for conducting recording and/or reproducing with respect to an information recording medium includes the above described beam irradiation apparatus of the present invention.
Furthermore, according to a further aspect of the present invention, there is provided a method for manufacturing an original disk for an information recording medium, which is a method for manufacturing an original disk for an information recording medium, the original disk being used to fabricate an information recording medium having minute concaves and convexes formed thereon. An original disk for an information recording medium is obtained via an exposure step of conducting an exposure processing on a photoresist layer coated on a substrate of the original disk, by using a beam irradiation apparatus including a beam generation source for generating at least either of an optical beam, an electron beam and an ion beam, and a focusing lens for focusing a beam emitted from the beam generation source, and a development processing step of conducting a development processing on the photoresist layer.
Furthermore, according to a yet further aspect of the present invention, there is provided a method for manufacturing an information recording medium, which is a method for manufacturing an information recording medium having minute concaves and convexes formed thereon. The method comprises the steps of manufacturing an original disk, manufacturing a stamper by using the original disk and forming an information recording medium by using the stamper.
In this case, the step of manufacturing an original disk includes an exposure step of conducting an exposure processing on a photoresist layer coated on a substrate of the original disk, by using a beam irradiation apparatus comprising a beam generation source for generating at least either of an optical beam, an electron beam and an ion beam, and a focusing lens for focusing a beam emitted from the beam generation source, and a development processing step of conducting a development processing on the photoresist layer.
As described above, in the focus control for controlling the focus position of the beam in the present invention apparatus, for example, a coarse adjustment and a fine adjustment, i.e., focusing control in the low frequency band and focusing control in the high frequency band are conducted for the focusing lens or the same lens system forming the focusing lens, by control mechanisms of equal to or more than two stages including first and second control mechanisms. As a result, the focus position adjustment is conducted accurately and stably.
In the present invention method, an original disk is manufactured by using such a present invention apparatus, and an information recording medium is manufactured by using this original disk. Therefore, an excellent information recording medium capable of conducting a high-density recording can be obtained.
In Japanese Patent Application Laid-Open No. 4-184722, there is disclosed such a configuration that a focusing servo is conducted by using a lens having a small diameter and an objective lens. In the present invention, however, the first control and the second control are conducted for a common lens. As a result, position setting i.e., focusing can be controlled more accurately and easily. Especially in an adjustment in the near-by field region, therefore, position setting i.e., focusing can be controlled with high precision and stability.